Cationic electrodeposition paint compositions excel in coating workability, and films formed thereof exhibit good corrosion resistance. Hence they are widely used as undercoat of electrically conductive metallic articles such as car bodies which require these properties.
Base resins which are used in cationic electrodeposition paint compositions normally are produced using organic solvent, and are blended in the form of organic solvent-containing resin solutions (varnishes). Resulting cationic electrodeposition paint compositions, therefore, become organic solvent-containing paint compositions.
Cationic electrodeposition paint compositions also are usually blended with organic solvent of good hydrophilicity/hydrophobicity balance, such as ethylene glycol monobutyl ether (boiling point, 171° C.), ethylene glycol monohexyl ether (boiling point, 208° C.), propylene glycol monopropyl ether (boiling point, 150° C.), cyclohexanone (boiling point, 145° C.) and the like, for such purposes as improving water dispersibility of the resin component to increase paint stability and improving film thickness retention with time, finished appearance of coating film and electrocoatability on galvanized alloy steel sheet.
On the other hand, use of these organic solvents is restricted in recent years for consideration of environments, by volatile organic compound (VOC) regulation or harmful atmospheric pollutants (HAPs) regulation.
However, when volatile organic compound content of cationic electrodeposition paint composition is reduced, such drawbacks as deterioration in film thickness retention with time, finished appearance of coating film or electrocoatability of galvanized alloy steel sheet may occur.
JP Sho 63 (1988)-92637A discloses electrodeposition paint containing cationic epoxy resin which is produced by adding organic acid and water to a resin obtained through reaction of a composition comprising diglycidyl ether of at least one kind of polyol and diglycidyl ether of at least one kind of dihydric phenol, with at least one kind of dihydric phenol, to convert the oxysilane groups in said resin to cationic groups. Coating film formed by electrocoating the electrodeposition paint comprising the cationic epoxy resin, however, shows insufficient corrosion resistance.
JP 2003-221547A discloses cationic electrodeposition paint containing as the base resin xylene formaldehyde resin-modified, amino group-containing epoxy resin formed through reaction of an epoxy resin having epoxy equivalent of 180-2,500 with xylene formaldehyde resin and amino group-containing compound. The cationic electrodeposition paint containing the xylene formaldehyde resin-modified, amino group-containing epoxy resin as the base resin, however, may show defect in either of film thickness retention, finished appearance of coating film and electrocoatability on galvanized alloy steel sheet, when its organic solvent content is decreased for VOC reduction.
JP Hei 8 (1996)-245750A discloses cationic electrodeposition paint containing an epoxy resin produced from diglycidyl ether of polyether polyol, which has an average epoxy equivalent of 350-5,000, or an epoxy resin produced from glycidyl ether of partially capped polyether polyol, which has an average epoxy equivalent of 300-5,000. A large amount of organic solvent must be added, however, for the cationic electrodeposition paint containing such an epoxy resin to exhibit favorable electrocoatability of galvanized alloy steel sheet, and the purpose of reducing volatile organic compound cannot be accomplished.
JP 2001-3005A discloses a low VOC-containing paint excelling in film-forming property, electrocoatability of galvanized alloy steel sheet and corrosion resistance, which contains alkylene-type polyether polyol such as polymethylene glycol, polyethylene glycol, polypropylene glycol or polybutylene glycol, or polyether polyol such as bisphenol alone or aromatic ring-containing polyether polyol obtained through reaction of bisphenol with glycol, as an additive. Also JP 2006-274234A discloses high stability electrodeposition paint which can form coating film free of pinholes and of excellent corrosion resistance when electrocoated on rust-preventive steel sheet, of which volatile organic solvent (VOC) content is reduced by blending specific polyether compound having a molecular weight not more than 1,000 as an additive. When a large amount of the additive disclosed in JP 2001-3005A or JP 2006-274234A is added to electrodeposition paint, however, corrosion resistance of formed coating film may drop, or the paint stability may drop when mechanical load is exerted on the electrocoating bath over a prolonged period.